Depending on the required selected power class, mobile telephones emit transmission power levels of up to 2 W via the transmitting antenna. Particularly in the edge of the supply area of a radio cell or when transmission is interfered with by multipath propagation, the network system administration for the mobile telephone will often choose the highest power class in order to set up and maintain the connection.
If the antenna is in the form of a (shortened) monopole which is fitted externally on the telephone and if there is no interference in the environment in the immediate near field of the antenna, omnidirectional emission is achieved from the mobile telephone, that is to say emission in all directions. Since mobile telephones are normally used against the ear and against the head when setting up and maintaining communication connections, this equally results in undesirable radio-frequency irradiation of the head together with the organs located in it, for example the brain and the eyes. In this case, the assessment principle for the irradiation is the conversion of the radio-frequency energy from the electromagnetic field into body heat, which is indicated in the so-called specific absorption rate (SAR). Depending on the intensity of the irradiation, this leads to a local temperature increase in the head, associated with possible damage to the tissue (microwave effect). Furthermore, so-called non-thermal effects are also currently being discussed, whose possible effects on human health it has not, however, yet been finally possible to verify scientifically.
The majority of the previous methods for reducing the radiation load have been based on a specific configuration of the antenna and its surrounding area so as to jointly optimize the antenna structure and the mounting board with respect to reducing the radiation passing through the user. An integrated planar antenna element is normally used for this purpose, which is fitted on the rear earth side of the mounting board and thus, due to the screening effect of the board, preferably emits away from the head. If the antenna and the board are carefully designed and optimized jointly, together with the other parts of a mobile telephone such as the housing shell, the display and the keypad, it is possible to reduce the power emitted in the direction of the head. In this case, the main lobe direction remains permanently set.
The greatest problem with this method is often the immediate area surrounding the antenna: for example, the earthing configuration, which is normally too small and is thus poor for the frequency band, which is in the Gigahertz range, in the form of the mounting board in the mobile appliance results in major interaction between the antenna and the area surrounding the antenna or the mobile appliance. This interaction means that any change in the area surrounding the mobile appliance, which also includes the way in which the mobile appliance is fixed, in some circumstances may even lead to an increase in the radiation load, since the entire telephone then acts as an antenna and can contribute to emission in any direction, that is to say in particular it can even lead to increased emission in the direction of the user. The omnidirectional radiation characteristic of a mobile appliance normally changes considerably as soon as it is placed against the ear. The process of optimizing the antenna structure in conjunction with the configuration of the mobile appliance is thus carried out only for a single static case, in general with the mobile appliance being held in the ideal manner.
Patent Specification U.S. Pat. No. 6,484,015 B1 discloses an apparatus which envisages the use of a directional antenna with a main lobe direction parallel to the ground or two or more physically separate directional antennas with different main lobe directions parallel to the ground in a mobile radio. In the case of a directional antenna facing away from the user, the purpose of this antenna apparatus is to improve the efficiency as the ratio between the used power and the emitted power, and to reduce the radiation load for the user of the mobile station. The document relates to static directional antennas, whose directional characteristics are not variable.
The Patent Specification U.S. Pat. No. 6,489,465 B1 describes a method in which the user of a mobile telephone is requested to change his position if the antenna is poorly positioned. The mobile telephone antenna does not have a directional characteristic.
German Laid-Open Specification DE 101 23 107 A1 describes a mobile telephone which uses a directional antenna whose main lobe direction can be aligned in the direction of the fixed station by measuring the received power. The total radiation power can be reduced by the directional emission. However, in this method, interference influences can adversely affect the optimum antenna alignment as a result of the directional power measurement for location of the fixed station.